Mixing apparatus and method

ABSTRACT

A mixing apparatus and method for mixing materials, such as plastics, comprising an outer stator member and an inner rotor member mounted for rotation and axial movement therein. The apparatus has a charging end and an exit end; and both the rotor and stator have cooperating screw threads along their lengths for feeding material through the apparatus. An extensive mixing section, defined by the cooperating threads of the rotor and stator, extends from the charging end toward the exit end of the apparatus. In this section, the cumulative groove depth of the rotor and stator is progressively smaller at locations nearer the exit end of the apparatus. An intensive mixing section is provided at the exit end of the apparatus for further mixing the material after it passes through the extensive mixing section. In the extensive mixing section, the interface defined between the threads on the rotor and stator forms a cylindrical surface while the interface defined between the threads in the intensive mixing section forms a conically tapered surface. Axial relative adjustment of the rotor and stator changes the clearance between the rotor and stator in the intensive mixing section to vary the extent of mixing but such adjustment does not have any appreciable effect on the mixing in the extensive mixing section of the apparatus.

United States Patent Stansfield [451 Aug. 22, 1972 [54] MIXING APPARATUS AND METHOD [72] Inventor: Manfred D. Stansfield, Edison, NJ. [57] ABSTRACT [73] Assignee: Sterling Extruder Corporation, A mlxmg R m method for .nuxmg a South P1 ainfi e1 d Ni such as plast1cs, comprising an outer stator member and an inner rotor member mounted for rotation and [22] Filed: Oct. 26, 1970 axial movementtherein. The apparatus has a charging end and an exit end; and both the rotor and stator [21] Appl' 83959 have cooperating screw threads along their lengths for Related US Application Data feeding material through the apparatus. An extensive mixing section, defined by the cooperating threads of C0ntmuaI10n-mpart 0f Se'r- 69,350, p the rotor and stator, extends from the charging end 1970, abandoned toward the exit end of the apparatus. In this section, the cumulative groove depth of the rotor and stator. is [52] US. Cl ..259/10, 260/94.9 progressively smaller at locations nearer the exit end [51] Int. Cl. ..B0lf 7/02 of the apparatus. An intensive mixing section is pro- [5 8] Field of Search ..259/9, 10, 97, 3; 18/125 A, vided at the exit end of the apparatus for further mix- 18/125 B, 125 J; 260/949 C, 94.9 CB ing the material after it passes through the extensive mixing section. In the extensive mixing section, the in- [56] References Cit d terface defined between the threads on the rotor and stator forms a cylindrical surface while the interface UNITED STATES PATENTS defined between the threads in the intensive mixing 2 744 287 5/1956 Parshall ..259/9 a micany taperedsurface- Axia' 2754542 7,1956 Henning 259/9 tive ad ustment of the rotor and stator changes the 28O1237 7/1957 Cle "5 clearance between the rotor and stator in the intensive 3IO2694 9/1963 -gi f 259/9 mixing section to vary the extent of mixlng but such Primary Examiner-Robert W. Jenkins Attorney-Pennie, Edmonds, Morton, Taylor & Adams It I iii adjustment does not have 'any appreciable effect on the mixing in the extensive mixing section of the apparatus.

47 Claims, 2 Drawing Figures INTENSIVE MIXING SECTION STAGE VENTING SECTION MIXING APPARATUS AND METHOD RELATED APPLICATION This application is a continuation-in-part application of application, Ser. No. 69,350, filed Sept. 3, 1970, now abandoned, entitled Mixing Apparatus and Method.

BACKGROUND OF THE INVENTION The present invention relates to a mixing apparatus suited for mixing plastic materials. Examples of prior art mixing apparatus are found in US. Pat. Nos. 2,910,726, 3,102,716 and Re 26,147. The machines disclosed in these patents generally include a stator and cooperating rotor both of which are helically threaded for passing material from one end of the apparatus to the other and for subjecting the material to various types of mixing and blending operations during its passage through the apparatus.

In conventional mixing apparatus such as described in the above-mentioned patents, a distinction is made between extensive mixing of the material or materials and intensive mixing. Extensive mixing generally includes the dispersal of materials into a homogeneous mixture while intensive mixing is more directed to a shear working of mixed materials. Where extensive mixing is desired, an apparatus such as disclosed in the above-mentioned US. Pat. No. 3, 1 02,7 16 is employed. To create this extensive mixing, the lands of the threaded rotor and stator are generally minimized in size so that there is a minimum of closely spaced surfaces on the rotor and stator passing each other during rotation of the rotor. A construction of the type shown in US. Pat. No. 3,102,716 produces maximum mixing of the materials while maintaining a minimum shearing of the materials.

Where it is desired to subject the material to intensive mixing, an apparatus such as shown in the abovementioned US. Pat. No. 3,164,375 is employed. Here, the lands of the cooperating rotor and stator are relatively wide and positioned with a close clearance so as to create shearing of the material as it is passed therebetween. Depending on the material being mixed and the results desired, either an extensive mixing or intensive mixing apparatus will be used.

With the machines disclosed in the above-mentioned patents, such characteristics as the construction of the threaded surfaces and temperature and speed restrictions limit the materials which may be mixed therein. These machines are not, for example, well suited for mixing plastic materials including many polymeric materials where it is desired to improve their molecular weight distribution. Machines of the types described above are generally unsuited for this purpose because they cannot mix the polymeric materials with a progressively stepless increase in shear rate up to predetermined maximum which can be varied.

In addition to the above-described extensive and intensive mixing machines, prior art machines for handling polymeric materials have included single and twin screw extruders, batch and continuous internal mixers and milling machines. Various ones and combinations of these machines have been used for particular processing operations; but they have not been completely satisfactory because of either cost of the installation, operation and maintenance, room required for the machine layout and/r inherent processing limitations. For example, in making master-batches or color concentrates, prior techniques have generally employed batch or continuous internal mixers in conjunction with a mill or extruder. Alternatively, either a twin screw extruder or mill has been used by itself. These prior art techniques are not completely satisfactory in that they either result in high capital investment for the required process line, require a number of separate pieces of equipment and large floor space and/orgive riseto high labor and maintenance costs. Also, when it is desired to change from one processing operation involving certain materials to. a different operation, it is generally necessary to use different machine layouts or change extruder screw configurations.

Prior art techniques have also presented difiiculties in compounding and extruding certain polymeric materials and required specially built single or twin screw extruders. Conventional twin screw extruders can generally handle a greater number of extrusion and compounding operations and also a greater variety of let down or diluting operations than can single screw extruders. With twin screw extruders,.however, the cost is much greater and if the resin is changed, the twin screws have to be changed or in some cases rebuilt by assembling components in difi'erent combinations. This not only requires stockpiling of different screw components but also necessitates extended shut down periods when a change-over is made. Even with specially built screw extruders there are still certain difficult extrusion, compounding and let down operations that cannot be done economically or in the pounds per hour desired.

Also, prior art machines presently available for compounding and extruding shear and heat sensitive materials are not completely satisfactory. This is because these machines do not have the capability of controlling the shear rates and temperatures to which the material is subjected during the processing opera tion. Accordingly, special precautions must be taken to prevent degradation, premature reaction or the oc-.

curance of some other undesired physical or chemical change. In the past, processing of these materials has required the use of continuous fluxing mixers followed by a hot melt extruder or mill for transforming the product into pellet or diced form. Alternatively, specialized extruder screw configurations have been required.

SUMMARY OF INVENTION 'Ihe present invention relates to an apparatus for mixing plastic materials which does not have the operating limitations present in prior mixing apparatus. The present invention also relates to a method of mixing various polymeric materials which previously have required specialized equipment.

Generally, the apparatus of the present invention includes a threaded stator and cooperating rotor rotatably and axially movable relative to the stator. Two separate mixing sections are provided along the length of the apparatus, one for extensively mixing the material and the other for intensively mixing it. The first extensive mixing section extends from the charging end of the apparatus, toward its exit end. In this section, the opposed lands on the rotor and stator are relatively narrow to produce the desired extensive mixing.

I In addition, the cumulative groove depth between opthe pitch, pitch angle and number of flights, are chosen to produce the desired'mixing of the particular materi-' als being fed through theapparatus'.

Disposed downstream of the extensive mixing sectionis the intensive mixing section. Here, the lands of the oppositely disposed stator and rotor are relatively wide and closely spaced to efiect shearing of the material passing therebetween. .In the intensive mixing section of the apparatus, the rotor and stator are conieffect a breaking of the polymer chains and cally tapered so that adjustment in the shear rate to which the material is subjected can be varied as dictated by the particular material being mixed.

Although the interface between the rotor and stator in the intensive mixing section is conically tapered, the interface between the lands of the rotor and stator in the extensive mixing section defines a cylinder. With this construction, axial adjustment of the rotor may be made to vary the shear rate to which the material is subjected in the intensive mixing section and such adjustment will have a negligible'efi'ect on the mixing operation in the extensive mixing section.

,By adjusting the axial position of the rotor relative to the stator and by varying the running speed of the machine, the plastic materials may. be subjected to shear rates at the exit end of the machine ranging anywhere from 50 to 5,000 reciprocal seconds or even below and above these values with certain materials. Also, with the conical and cylindrical interfaces and the continuously decreasingcumulative groove depth in the extensive mixing section, the apparatus, in effect,

. pounded'and extruded by simply adjusting the operating conditions of the machine and without requiring multiple machines or different screw configurations of the rotor and stator. Also, the apparatus of the present invention may be used to improve both the visual and structural properties of many polymeric materials by making the molecular weight distribution more uniform. More particularly, the extensive mixing of the plastic materials immediately preceding the final intensive mixing may be controlled by choosing appropriate land and groove configurations and by controlling the heat transfer to and. from the material to properly prepare the material for its final passage through the intensive mixing section. With such preparation of the materia and final mixing at high shear rates, the material exiting from the apparatus will have the desired uniform molecular weight distribution.

With the presentinvention, it is possible, for exampie, to reduce gel content in low density polymer films so as to produce clear films and thus avoid the creation of unacceptable fish eyes. The method and apparatus of the present invention may also be used for improving as those containing long entwined polymer'chains or;

crystallized resins. The stepless mixing be usedlto tion of the crystallinity of the resins.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la is a schematic view inj cross section of the 1 A charging-end 0f the apparatus showing part of the extensive mixing section; and 1 FIG. lb is aschematic view in cross section of the exit end of the machine showing the remainder of the extensive mixing section and intensive mixing section thereof. 1 I

DESCRIPTION OFTHE PREFERRED I EMBODIMENT In the drawings, the mixing apparatusis shown as comprising inner screw member lin the formof a I rotor and outer screw member 2- "forming a stator.- The rotor is mounted for rotation within the stator, the] T drive for this being from the motor 3. The rotor is also mounted for axial adjustment within the stator by means of a structure generally'shown at 4.

FIG. 1a shows the charging end of the apparatus. Here, material to be mixed is fed through an-in'let 5 in the stator. The feeding is by gravity fromthe metering mechanisms 6 and 7. One metering mechanism may, for example, feed a resin while .the other feeds a pigment or filler to be mixed with the resin. As the material to be mixed is through'the charging end, it is fed along the axis of the machine under the influence of the cooperating helical threads 8 and '9 formed on the rotor and stator members. After the material has been subjected to the desired mixing and working operations as more fully described below, it passes out the exit end of the apparatus. Here, a suitable die 10 having an extrusion orifice is provided. r The apparatus of the present invention is constructed with both an extensive mixing section and an intensive section. withreference-to the drawings, the extensive mixing section extends from the charging end of the apparatus toward the exit end and is divided into sixmixing stages designated I-VI. The intensive mixing section, on the other hand, is disposeddownstream offacilitating removal of gasses, water and other volatiles from the material. If desired, a number-of separate venting sections may be provided for increasing the quantity of gasses, water and other volatiles withdrawn from the material. The apparatus also includes zoned heaters and cooling coils designated, respectively, at 14 and 15 disposed along the length of the stator structure for controlling the temperature of the material being fed through the apparatus. For cooperating with the heaters and cooling coils, the rotor is cored at 13 for a other structural properties of polymeric materials, such passage of cooling water.

the reduc- Completing the description of the basic structure of the apparatus, the stator 2 is constructed from a number of axial segments. These segments correspond in length to the length of the various Stages I-VI of the extensive mixing section and to the length of the stages in the intensive mixing section. This construction facilitates the different internal threading thereof as more fully described below.

In accordance with the teachings of the present invention, the helical threading of the rotor and stator in the extensive mixing sections is defined by land and adjacent groove convolutions with the lands being narrower than the grooves. Preferably, the lands are as narrow as structurally possible so that the opposing surfaces defined thereby will be at a minimum. In addition, the lands on both members are formed to lie in concentric cylindrical surfaces and produce a cylindrical interface designated by the dashed line 17.

Each stage of the extensive mixing section advantageously has a certain helical thread arrangement depending on the mixing operation to be performed. Also, the threading of the rotor and stator at the charging end and in the venting section of the apparatus is constructed for cooperating with the other threaded sections to effect the desired mixing and working of the material being fed through the apparatus.

Referring to FIG. la, the rotor at the charging end of the apparatus is shown as being deeply grooved while the inner surface of the surrounding stator is smooth surfaced. Advantageously, the groove depth of the rotor at the charging end of the apparatus is at a maximum so as to permit introduction of a wide range of materials without requiring a hopper stuffer. Also, with the rotor thread at the charging end having the helix angle shown in FIG. la, the introduction of a maximum amount of material is facilitated. In construction,'the pitch of the rotor thread at the charging end of the apparatus is advantageously one diameter of the rotor and the threading is of the single start configuration. With this construction, plastic granules or powder, pigment or fillers in proportions up to 50 percent by weight, and in some cases even higher, can be metered separately into the charging end of the apparatus or added as a preblend.

After the material is dropped into the charging end of the apparatus, it is fed along the rotor to the extensive mixing section of the apparatus and through the various Stages IVI thereof. As shown in the drawings, the first four stages are located upstream of the venting section while the next two stages are downstream thereof. In the upstream stages, the extensive mixing generally produces a melting and mixing of the material and the wetting out of any additives. In the downstream stages, the material is subjected to further mixing to properly prepare it for intensive mixing in section 1 1.

In Stage I of the extensive mixing section, the rotor and stator are constructed to readily accept the material from the deep grooves at the charging end and to start extensive mixing, melting and encapsulating of any pigment or fillers. More particularly, the grooves of the rotor and stator are constructed so as to pass the material from the rotor to the stator. For this purpose, the groove depth in the rotor is decreased from a maximum to a minimum while the depth of the opposite groove of the stator is increased from a minimum to a maximum. Also, the rotor in Stage I is provided with a second flight of threads of the same pitch as the thread in the charging end of the apparatus while the stator has a two start thread configuration. Finally, the cumulative groove depth of the rotor and stator is made progressively smaller at locations nearer the exit end of the apparatus. The decreasing nature of the cumulative groove depth in Stage I is shown by the non-parallel relationship of the extended lines 18 and 19 drawn tangent to bottom surfaces of the grooves in the rotor and stator.

After passing through Stage I, the material enters Stage II where it is transferred back from the stator to the rotor. For this purpose, the stator is provided with grooves of decreasing depth while the rotor is provided with grooves of increasing depth. Also, for providing a more intensive working of the material in Stage II, the helix angle of the rotor is decreased and a one or two start threaded configuration is used while the stator retains its two start threaded'configuration. As in Stage I, the cumulative groove depth of the rotor and stator in Stage II progressively decreases.

In Stage III the material is once again passed from the rotor to the stator, the groove depth of therotor varying from a maximum to a minimum and that of the stator from a minimum to a maximum. In Stage III, the rotor continues with the threads having the same helix angle as provided in Stage II. The stator, on the other hand, changes from a two start to a one start configuration to increase the intensity of the mixing. As with the previous stages, the cumulative groove depth again decreases to further intensify the mixing action on a gradient scale. t

In Stage IV, the material is, once again, passed back from the stator to the rotor by using the appropriate groove configurations while the decreasing of the cumulative groove depth is continued for further intensifying of the mixing of the material. In Stage IV, the helix angle in the stator remains the same as it was in Stage III. In the rotor, however, the helix angle is increased for optimum transport of the material and proper feeding into the ventingsection.

The particular configuration of the threading of the rotor and stator in Stages I-IV may be changed for different materials and for producingdifierent mixing operations. In each case, however, the interface formed by the lands of the rotor and stator willlie along a cylindrical surface and the cumulative groove depth will progressively diminish at locations nearer the exit end of the apparatus. For example, after the material has been melted and any pigment orfillers have been encapsulated in Stages I and II, the intensity of the mix may be increased or decreased, as desired, by changing the configuration of the cooperating threads in Stages Ill and IV so as to produce a relativelyhomogeneous mix and assure that the additives become completely wetted out.

After the material passes through Stage IV, it may be subjected to venting depending on whether it is desired to remove any entrapped air, moisture" or other volatiles. The venting may be directly to atmosphere or may be effected by the vacuum pump 12. Where a venting section is used, the groove depth of the rotor in this section is advantageously at a maximum commensurate with proper mixing in the stages on either side.

"By employing a venting section in the apparatus, not

,air and moisture allows the shear work produced downstream of the venting section to be transmitted directly to the pigment, fillers and any agglomerates in the material without the cushioning effects which trapped air, moisture and other volatiles would tend to create.

In Stage V, the groove depth of. the rotor at the end immediately adjacent the venting section is the same as the groove depth of the rotor in the venting section.

This groove depth, however, progressively varies from.

a maximum tov a minimum toward the downstream end .of Stage V while the depth of the opposite grooves of the stator varies inversely from a'minimum to a maximum. As withthe preceding stages of the extensive mixing section, the cumulative groove depth of the rotor and stator progressively diminishes in Stage V. The groove depth in the stator at-the downstream end of Stage V is dictated by the maximum shear work and pumping efliciency desired. The shallower the cumulative groove depth at the downstream side of Stage V, the higher the pumping efiiciency attained. An increase in pumping efficiency is desired preparatory to feeding of the material into the intensive mixing section of the apparatus.

After the material: passes through Stage V, it is fed into the final Stage. VI of the extensive mixing section of the apparatus. Here, the grooves of the rotor and stator are configured to once again pass the material back from the stator to the rotor. Also, the cumulative groove depth in Stage Vl progressively diminishes toward the exit end of the apparatus.

With the intensive mixing section constructed for varying the shear rate between about 50 to 5,000 reciprocal seconds, the cumulative groove depth of Stage VI immediately upstream of the intensive mixing section may be of such a value so as to provide a shear rate which is about equal to the lower range of shear rates producible in the intensive mixing section. More particularly, the cumulative groove depth in Stage'VI immediately upstream of the intensive mixing section will be equal to or greater than the maximum clearance obtainable between the cooperating surfaces of the rotor and stator in the intensive mixing section. With this construction, the material can be subjected to shear rate which progressively increases to a value of from about 30 to about 500 reciprocal seconds immediately preceding intensive mixing, Then in the in-, tensive mixing section, the apparatus may be set to subject the material to the final shear rate desired ranging anywhere from about 50 to 5,000 reciprocal seconds.

After the material has been fed through all of the six stages of the extensive mixing section, it is fed directly I of the rotor and stator are located on closely spaced,

conically tapered surfaces. Also, the groove construction advantageously varies twice between a minimum to a maximum in both the rotor and stator to effect transfer of the material back and forth between the rotor and stator. Instead of the groove construction shown, it may, in some situations, be desirable to provide the intensive mixing sectionwith smooth, conical or otherwise tapered surfaces. In either case, the large cooperating surfaces provided produce a good intensive mixing of the material in the final mixing stage of the apparatus. The material as it leaves the intensive mixing section is advantageously extruded through a suitable die 10 such as used with extruders.

With the cylindrical interface construction in the extensive mixing section and the conicallytapered interface in the intensive mixing section section, final intensive mixing'of the material may be effected at a wide range of shear rates without adversely affecting the mixing in the extensive section. Variation in the mixing of the intensive mixing section is-controlled by adjusting the axial position of the rotor relative to the stator.

With the presentlyv preferred construction, sufficient relative axial movement is provided-for effecting variances in the shear rate generally from 50 to 5,000 reciprocal seconds. With certain materials, however,

lower or higher shearrates may be produced. The shear rate to which the material is subjected in the intensive mixing section is also controlled by controlling the speed of rotation of the rotor within the stator, with higher speeds increasing the shear rates. Although changes in the axial clearance of the roto and stator will have an efi'ect on the intensive mixing section of the apparatus, it will nothave any appreci a,

ble effect on the mixing in the extensive mixing section due to the cylindrical interface between the rotor and stator in this section and due to the narrow lands. Axial movement willnot change the clearance between the rotor and stator in the extensive mixing section. Also, with the construction of the apparatus as described of the rotor and stator which bring the bulk flow of the material, layer by layer, in. contact with the heat transfer surfaces of the rotor and stator. Thus, good turbulence can be introduced into relatively viscous materials without causing excessive increase in temperature during the extensive mixing.

The zonedheaters and cooling coils in the extensive mixing section can be adjusted to control the heating and cooling of the material being fed through the apparatus. Different materials require different cooling will cause further homogenization and breakdown of any aggregate in the material such by pigments or fillers and properly prepare the material for'its final intensive mixing. At the same time, the shear work is'being increased, the progressively decreasing cumulative depth of the groove in Stages V and VI produces the desired increase in the shear rate to the value approaching the low end of the range obtainable in the intensive mixing section. Alternatively, where shear work and/or shear rate is not to be increased, as where breakdown of aggregates or destruction of fibrous fillers is, not desired, the threaded configuration of the rotor and stator in Stage V and VI can be changed and the heating or cooling of the material appropriately adjusted to produce the desired results.

With the construction of the apparatus as described above, the material fed into the inlet is taken up by the rotor helix and peeled off in a layer by layer fashion by the stator helix as the groove depth of the rotor decreases and that of the stator increases. As this transfer is occurring, the vortexing action taking place in the material in the rotor grooves becomes smaller while that in the stator become larger. Thus, all material on the inside of the vortex is transferred to the outside, and vice versa, as many times as the material is passed between the rotor and stator. This insures that all material in passing through the various stages of mixing comes in close proximity to both the rotor and stator thus permitting good heat transfer between all of the material and the temperature controlled surfaces of the rotor and stator.

With the present invention, a single piece of equipment may be used in the production of various color concentrates and masterbatches. Pigments may be added to resins either in low concentrations of a fraction of a per cent, intermediate concentrations or high concentrates. It is only necessary to alter the feeder settings, temperature profiles, running speeds and clearance between the rotor and stator to handle. a large variety of polymeric materials. Resins such as ABS, Nylon, low density and high density polyethylene, general purpose or high impact styrene, PVC, polypropylene and polycarbonate can be run in the same machine by simply varying the operating conditions thereof.

predispersed, for example, with carbon black (medium channel black or fine furnace black) or other pigments may be mixed with additional resin in let down operations. No special extruders or combinations of mixers and extruders is required. This is also true with respect to the melting and extruding of materials such as high density linear polyethylene and the melting and compounding of different combinations of resins having different physical properties. With the present invention, the processing of all of these materials may be carried out in the same apparatus; and the amount of shear work to which the material is subjected can be controlled whether the system requires a high shear rate and high temperature treatment or a low shear rate and low temperature treatment.

Controlling the shear work is especially important with resin systems where instead of the shear work increasing with an increasing shear rate, it decreases due to the viscosity going down more rapidly than the shear rate goes up. In such situations, the ability to run the low shear rates with low temperatures is of paramount importance. Thus, by cooling of the molten resin to just above the softening temperature, the shear work can'be optimized even at minimum shear rates.

Due to the excellent heat transfer capabilities of the apparatus and method of the present invention and due to the fact that the intensive mixing may be varied over a wide range of shear rates, it is possible to compound and extrude many shear sensive materials which in the past required specialized equipment. For example,

In addition, glass filled polypropylene pellets may be produced with the apparatus and method of thepresent invention by mixing'chopped glass fiber filaments with the polypropylene in either powdered or pellet fonn. The glass fibers have to remain in tact to impart the desired strength to the finished pellets; and this is possible with the present invention since the shear rates to which the material is subjected can be limited. Another shear sensitive material which may be processed in accordance'with the method of the present invention is rigid PVC in either powdered or pellet form. This material can be extruded directly into pipe or sheet form. By controlling the heat transfer and shear rates of the material, degradation is prevented.

The processing of the various polymeric materials as described above is generally done with the material received in a dry, solid state. Typically, the material may be in either granular, powder, flake, diced or pellet form. The apparatus and method of the present invention is also adaptable for mixing and compounding molten polymeric materials with solid and/or liquid additive materials. This is particularly advantageous where the polymeric material which is to be combined with a particular additive is, itself, the output of a prior processing operation.

With the present invention, molten material produced in one processing operation can be fed directly into the mixing apparatus and there combined with other additive materials in either solid or liquid form. For example, molten resins from a reactor may be fed directly to the apparatus of the present invention for the addition of color pigments, anti-oxidents, flame retardance and so forth. Also, liquid plasticizers, color pigments and other additives can be added and properly mixed with PVC and Nylon without causing decomposition of the material. Similarly, hot melts such as adhesives and coatings used for paper, cardprocessing operations can be performed on various polymeric materials and that all of these operations can be performed by a single'apparatus. I-Ieretofore, these processing operations could only be performed with the specialized equipment and sometimes only at considerable expense.

I claim:

' 1. In a mixing apparatus having an inner screw member and a cooperating relatively rotatable outer screw member disposed thereabout, a material charging end and an exit end, the improvement wherein:

a. said inner and outer screw members each have an extensive mixing section extending from the charging end of the machine toward the exit end thereof with at least one helical thread defined by land and adjacent groove convolutions, l. lands being narrower than the grooves with the lands of said inner screw member located on a first surface and the lands of said outer screw member located on a second surface disposed in radially opposite cooperating alignment with said first surface,

2. the depth of opposite grooves in the inner and outer screw members varying at least once, one from a minimum to a maximum and the other from a maximum to a minimum along at least a portion of said extensive mixing section, to pass the'material from one member to the other as it is fed axially through said machine, and

I 3. the cumulative depth of the opposite grooves being progressively smaller at locations nearer the exit end of the machine along said portion of the extensive mixing section. I

2.The improvement in a mixing apparatus according to claim 1 wherein:

a. said inner and outer screw members each have an intensive mixing section at the exit end of the machine with a major part of the opposite peripheries of said inner and outer screw members located in third and fourth surfaces, respectively, disposed in radially opposite cooperating alignment with each other. I 3. The improvement in a mixing apparatus according to claim 2 wherein: p 4

a. said first and second surfaces are cylindrical; and b. said third and fourth surfaces are tapered. 4. The improvement in a mixing apparatus according to claim 3 wherein: v

a. said third and fourth surfaces are conically tapered toward the exit end of said machine. v 5. The improvement in a mixing apparatus according to claim 4 wherein:

a. said inner and outer screw members are movable axially relative to each other to vary the clearance between said third and fourth surfaces. 6. The improvement in a mixing apparatus according to claim 5 wherein:

a. the inner and outer screw members in said intensive mixing section each have at least one helical thread defined by land and adjacent groove convolutions,

l.- said lands being generally wider than said grooves and located in said third and fourth surfaces, and I 2. the depth of opposite grooves in the inner and outer. screw members varying, one from a a. the opposite groove depthsin said extensive mixing section vary a plurality of times between minimum and maximum to pass material at least said intensive mixing section is equal'to or greater I than the maximum clearance-obtainable between saidthird and fourth surfaces.

9. The improvement in-a mixing apparatus according to claim 5 wherein: v

a. said third and fourth surfaces are movable relative to each other to vary the shear rates obtainable in the intensive mixingsection over a predetermined range;and m b. the cumulative groove depth of said inner and outer screw members immediately upstream .of said intensive mixing section is of value providinga shear rate .thereat about equal to the shear rate at the lower end of said range. 10. The improvement in a mixing apparatus according to claim 9 further including:

a. cooling means disposed immediately upstream of v said intensive mixing section for cooling the material -to increase its viscosity and the shear work to which it is subjected. V 11. The improvement in a mixing apparatus according to claim 5 wherein: p

a. the extensive mixing section includes a venting section intermediate its ends with the cumulativedepth of the grooves of the inner and outer screw members at said venting station being substantially uniform.

12. The improvement in a mixing apparatus according to claim 11 wherein:

a. the outer member at said venting station has a smooth inner surface; l

b. the' grooves in the inner member at said venting section are at a maximum for partial filling with the materialbeing fed through the machine.

' 13. The improvement in a mixing apparatus according to claim. 11 wherein:

- a. the depth of the opposite grooves in the inner and outer screw members on axially opposite sides of said venting section vary at least once, one from a minimum to a maximum and the other from a maximum to a minimum. a v v 14. The improvement in a mixing apparatus according to claim 13 wherein: Y

a. the cumulative depthof the opposite grooves upstream. of said venting section is continuously progressively smaller at locations nearer said venting station. 15. The improvement in a mixing apparatus according to claim 14 wherein:

a. the cumulative depthof the opposite grooves;

disposed immediately downstream of the venting section is progressively smaller at locations nearer said exit end.

16. The improvement in a mixing ing to claim 15 wherein:

outer screw members immediately upstream of I apparatus accord i a. the opposite groove depths upstream of said venting section vary a plurality of times between minimum and maximum to pass material at least once from the inner member to the outer member and back to the inner member with the material being in said inner member as it is fed axially into said venting section.

17. The improvement in a mixing apparatus according to claim 16 wherein:

a. the groove depth of the inner screw member is at a maximum at said venting section and immediately downstream thereof.

18. The improvement in a mixing apparatus according to claim 16 wherein:

a. the groove depth of the inner screw member is at a maximum at the charging end of the machine; and

b. the outer member at the charging end of the machine has a smooth inner surface.

19. The improvement in a mixing apparatus accord ing to claim 2 wherein: I

a. said inner and outer screw members in said intensive mixing section are movable toward and away from each other to vary the clearance between said third and fourth surfaces.

20. The method of mixing material including at least one polymeric material comprising the steps of:

a. heating said material to a temperature sufficient to effect melting thereof;

b. removing heat from the material after melting thereof to preclude an appreciable decrease in its viscosity and thereby optimize the shear work to which it is subjected; and

c. thereafter intensively mixing the material with a shearing action. I

21. The method according to claim 20 wherein:

a. said material is in a dry, solid, state as it is heated to effect melting thereof.

22. The method according to claim 20 wherein:

a. said polymeric material includes a first part consisting of a first polymeric material and a second part consisting of a second different polymeric material.

23. The method according to claim 20 wherein:

a. said material includes a polymeric material and a non-polymeric additive material.

24. The method according to claim 20 wherein:

a. said material includes a polyvinyl chloride and a plasticizer therefor.

25. The method according to claim 20 wherein:

a. said polymeric material is a high density linear polyethylene.

26. The method according to claim 20 wherein:

a. said material includes a first part consisting of a first polymeric material and a second part defining a masterbatch consisting of an additional amount of said first polymeric material with predispersed additive material therein.

27. The method according to claim 26 wherein:

a. said additive material is carbon black or a color pigment.

28. The method according to claim 20 wherein:

a. said material is a shear sensitive material which undergoes a physical or chemical change when subjected to combined shear rates and temperatures of predetermined values; and

b. the combined shear rates and temperatures to which said material is subjected during the entire mixing thereof is kept below said predetermined values.

29. The method according to claim 28 wherein:

a. said material includes a polyethylene and dicumyl peroxide.

30. The method according to claim 28 wherein:

a. said material includes precharged, expandable polypropylene.

31. The method according to claim 28 wherein:

a. said material includes polypropylene and chopped glass fibers.

32. The method according to claim 28 wherein:

a. said material is rigid polyvinyl chloride.

33. The method according to claim 20 including the step of:

a. extensively mixing said material to in part create said initial heating. I

34. The method according to claim 33 including the step of:

a. continuously moving said material ,along a predetermined path during said extensive and intensive mixing.

35. The method according to claim 34 including the step of:

a. progressively increasing the shear rate to which the material is subjected during at least the last part of said extensive mixing. 4

36. The method according to claim 35 wherein:

a. the shear rate to which the material is subjected during said intensive mixing is no less than the shear rate to which the material is subjected immediately prior to said intensive mixing.

37. The method according to claim 36 wherein:

a. the shear rate to which the material-is subjected during said intensive mixing is between about 50 to 5,000 reciprocal seconds.

38. The method of improving the molecular weight distribution of a polymeric material comprising the steps of: r

a. heating said material to a temperature sufficient to effect melting thereof;

b. decreasing the temperature of the material after melting thereof to increase its viscosity and the shear work to which it is subjected; and I c. thereafter intensively mixing the material at a high shear rate.

39. The method of mixing material including at least one polymeric material comprising the steps of:

a. extensively mixing said material at a temperature sufficient to effect melting thereof;

b. progressively increasing to a predetermined value the shear rate to which the material is subjected during at least the last part of said extensive mix ing; and

c. intensively mixing said material immediately after the extensive mixing thereof and at the same time subjecting the material to a shear rate about equal to or greater than said predetermined value.

40. The method according to claim 39 wherein:

a. the material is subjected to a shear rate increasing to a value from about 30 to about 500 reciprocal seconds during the last portion of said extensive mixing; and

b. the material is subjected to a shear rate of between abut'50'to 5,000 reciprocal seconds during said intensive mixing.

41. The method according to claim 40 including the step of:

a. continuously moving the material along a predetermined path during said extensive and intensive mixing.

42. The method according to claim 41 including the step of: l I

a. cooling the material immediatelyvprior to said intensive mixing to increase the shear work to which the material is subjected. Y

43. The method of improving the molecular weight distribution of a polymeric material comprising the steps of: Y g

a.extensively mixing said material at a temperature sufi'rcient to effect melting thereof;

. b. progressively increasing to a predetermined value the shear rate to which the material is subjected during at least the last part of said extensive mixing; and I c. intensively mixing said material immediately alter the extensive mixing thereof and at the same time subjecting the material'to a shear rate about equal to or greater than said predetermined value.

44. The methodof mixing molten polymeric material and solid or liquid additive material comprising the steps of:

a. combining said molten polymeric material and additive material; b. removing heat from the material after melting thereof to preclude an appreciable decrease in its viscosityand thereby optimize. the shear work to which it is subjected; and c. thereafter intensively mixing the material with a shearing action; 45. The method according to claim 44 wherein:

- a. said polymeric material is rendered molten. by a low density 

1. In a mixing apparatus having an inner screw member and a cooperating relatively rotatable outer screw member disposed thereabout, a material charging end and an exit end, the improvement wherein: a. said inner and outer screw members each have an extensive mixing section extending from the charging end of the machine toward the exit end thereof with at least one helical thread defined by land and adjacent groove convolutions,
 1. said lands being narrower than the grooves with the Lands of said inner screw member located on a first surface and the lands of said outer screw member located on a second surface disposed in radially opposite cooperating alignment with said first surface,
 2. the depth of opposite grooves in the inner and outer screw members varying at least once, one from a minimum to a maximum and the other from a maximum to a minimum along at least a portion of said extensive mixing section, to pass the material from one member to the other as it is fed axially through said machine, and
 3. the cumulative depth of the opposite grooves being progressively smaller at locations nearer the exit end of the machine along said portion of the extensive mixing section.
 2. the depth of opposite grooves in the inner and outer screw members varying at least once, one from a minimum to a maximum and the other from a maximum to a minimum along at least a portion of said extensive mixing section, to pass the material from one member to the other as it is fed axially through said machine, and
 2. the depth of opposite grooves in the inner and outer screw members varying, one from a minimum to a maximum and the other from a maximum to a minimum along at least a portion of said intensive mixing section.
 2. The improvement in a mixing apparatus according to claim 1 wherein: a. said inner and outer screw members each have an intensive mixing section at the exit end of the machine with a major part of the opposite peripheries of said inner and outer screw members located in third and fourth surfaces, respectively, disposed in radially opposite cooperating alignment with each other.
 3. The improvement in a mixing apparatus according to claim 2 wherein: a. said first and second surfaces are cylindrical; and b. said third and fourth surfaces are tapered.
 3. the cumulative depth of the opposite grooves being progressively smaller at locations nearer the exit end of the machine along said portion of the extensive mixing section.
 4. The improvement in a mixing apparatus according to claim 3 wherein: a. said third and fourth surfaces are conically tapered toward the exit end of said machine.
 5. The improvement in a mixing apparatus according to claim 4 wherein: a. said inner and outer screw members are movable axially relative to each other to vary the clearance between said third and fourth surfaces.
 6. The improvement in a mixing apparatus according to claim 5 wherein: a. the inner and outer screw members in said intensive mixing section each have at least one helical thread defined by land and adjacent groove convolutions,
 7. The improvement in a mixing apparatus according to claim 5 wherein: a. the opposite groove depths in said extensive mixing section vary a plurality of times between minimum and maximum to pass material at least once from the inner member to the outer member and back to the inner member as it is fed axially of said machine.
 8. The improvement in a mixing apparatus according to claim 5 wherein: a. the cumulative groove depth of said inner and outer screw members immediately upstream of said intensive mixing section is equal to or greater than the maximum clearance obtainable between said third and fourth surfaces.
 9. The improvement in a mixing apparatus according to claim 5 wherein: a. said third and fourth surfaces are movable relative to each other to vary the shear rates obtainable in the intensive mixing section over a predetermined range; and b. the cumulative groove depth of said inner and outer screw members immediately upstream of said intensive mixing section is of value providing a shear rate thereat about equal to the shear rate at the lower end of said range.
 10. The improvement in a mixing apparatus according to claim 9 further including: a. cooling means disposed immediately upstream of said intensive mixing section for cooling the material to increase its viscosity and the shear work to which it is subjected.
 11. The improvement in a mixing apparatus according to claim 5 wherein: a. the extensive mixing section includes a venting section intermediate its ends with the cumulative depth of the grooves of the inner and outer screw members at said venting station being substantially uniform.
 12. The improvement in a mixing apparatus according to claim 11 wherein: a. the outEr member at said venting station has a smooth inner surface; b. the grooves in the inner member at said venting section are at a maximum for partial filling with the material being fed through the machine.
 13. The improvement in a mixing apparatus according to claim 11 wherein: a. the depth of the opposite grooves in the inner and outer screw members on axially opposite sides of said venting section vary at least once, one from a minimum to a maximum and the other from a maximum to a minimum.
 14. The improvement in a mixing apparatus according to claim 13 wherein: a. the cumulative depth of the opposite grooves upstream of said venting section is continuously progressively smaller at locations nearer said venting station.
 15. The improvement in a mixing apparatus according to claim 14 wherein: a. the cumulative depth of the opposite grooves disposed immediately downstream of the venting section is progressively smaller at locations nearer said exit end.
 16. The improvement in a mixing apparatus according to claim 15 wherein: a. the opposite groove depths upstream of said venting section vary a plurality of times between minimum and maximum to pass material at least once from the inner member to the outer member and back to the inner member with the material being in said inner member as it is fed axially into said venting section.
 17. The improvement in a mixing apparatus according to claim 16 wherein: a. the groove depth of the inner screw member is at a maximum at said venting section and immediately downstream thereof.
 18. The improvement in a mixing apparatus according to claim 16 wherein: a. the groove depth of the inner screw member is at a maximum at the charging end of the machine; and b. the outer member at the charging end of the machine has a smooth inner surface.
 19. The improvement in a mixing apparatus according to claim 2 wherein: a. said inner and outer screw members in said intensive mixing section are movable toward and away from each other to vary the clearance between said third and fourth surfaces.
 20. The method of mixing material including at least one polymeric material comprising the steps of: a. heating said material to a temperature sufficient to effect melting thereof; b. removing heat from the material after melting thereof to preclude an appreciable decrease in its viscosity and thereby optimize the shear work to which it is subjected; and c. thereafter intensively mixing the material with a shearing action.
 21. The method according to claim 20 wherein: a. said material is in a dry, solid, state as it is heated to effect melting thereof.
 22. The method according to claim 20 wherein: a. said polymeric material includes a first part consisting of a first polymeric material and a second part consisting of a second different polymeric material.
 23. The method according to claim 20 wherein: a. said material includes a polymeric material and a non-polymeric additive material.
 24. The method according to claim 20 wherein: a. said material includes a polyvinyl chloride and a plasticizer therefor.
 25. The method according to claim 20 wherein: a. said polymeric material is a high density linear polyethylene.
 26. The method according to claim 20 wherein: a. said material includes a first part consisting of a first polymeric material and a second part defining a masterbatch consisting of an additional amount of said first polymeric material with predispersed additive material therein.
 27. The method according to claim 26 wherein: a. said additive material is carbon black or a color pigment.
 28. The method according to claim 20 wherein: a. said material is a shear sensitive material which undergoes a physical or chemical change when subjected to combined shear rates and temperatures of predetermined values; and b. the combined shear rates and temperatures to which said material is subjected during thE entire mixing thereof is kept below said predetermined values.
 29. The method according to claim 28 wherein: a. said material includes a polyethylene and dicumyl peroxide.
 30. The method according to claim 28 wherein: a. said material includes precharged, expandable polypropylene.
 31. The method according to claim 28 wherein: a. said material includes polypropylene and chopped glass fibers.
 32. The method according to claim 28 wherein: a. said material is rigid polyvinyl chloride.
 33. The method according to claim 20 including the step of: a. extensively mixing said material to in part create said initial heating.
 34. The method according to claim 33 including the step of: a. continuously moving said material along a predetermined path during said extensive and intensive mixing.
 35. The method according to claim 34 including the step of: a. progressively increasing the shear rate to which the material is subjected during at least the last part of said extensive mixing.
 36. The method according to claim 35 wherein: a. the shear rate to which the material is subjected during said intensive mixing is no less than the shear rate to which the material is subjected immediately prior to said intensive mixing.
 37. The method according to claim 36 wherein: a. the shear rate to which the material is subjected during said intensive mixing is between about 50 to 5,000 reciprocal seconds.
 38. The method of improving the molecular weight distribution of a polymeric material comprising the steps of: a. heating said material to a temperature sufficient to effect melting thereof; b. decreasing the temperature of the material after melting thereof to increase its viscosity and the shear work to which it is subjected; and c. thereafter intensively mixing the material at a high shear rate.
 39. The method of mixing material including at least one polymeric material comprising the steps of: a. extensively mixing said material at a temperature sufficient to effect melting thereof; b. progressively increasing to a predetermined value the shear rate to which the material is subjected during at least the last part of said extensive mixing; and c. intensively mixing said material immediately after the extensive mixing thereof and at the same time subjecting the material to a shear rate about equal to or greater than said predetermined value.
 40. The method according to claim 39 wherein: a. the material is subjected to a shear rate increasing to a value from about 30 to about 500 reciprocal seconds during the last portion of said extensive mixing; and b. the material is subjected to a shear rate of between about 50 to 5,000 reciprocal seconds during said intensive mixing.
 41. The method according to claim 40 including the step of: a. continuously moving the material along a predetermined path during said extensive and intensive mixing.
 42. The method according to claim 41 including the step of: a. cooling the material immediately prior to said intensive mixing to increase the shear work to which the material is subjected.
 43. The method of improving the molecular weight distribution of a polymeric material comprising the steps of: a. extensively mixing said material at a temperature sufficient to effect melting thereof; b. progressively increasing to a predetermined value the shear rate to which the material is subjected during at least the last part of said extensive mixing; and c. intensively mixing said material immediately after the extensive mixing thereof and at the same time subjecting the material to a shear rate about equal to or greater than said predetermined value.
 44. The method of mixing molten polymeric material and solid or liquid additive material comprising the steps of: a. combining said molten polymeric material and additive material; b. removing heat from the material afteR melting thereof to preclude an appreciable decrease in its viscosity and thereby optimize the shear work to which it is subjected; and c. thereafter intensively mixing the material with a shearing action.
 45. The method according to claim 44 wherein: a. said polymeric material is rendered molten by a separate processing operation prior to combining with said additive material.
 46. The method of mixing polymeric material and a liquid additive material according to claim 44 wherein: a. said polymeric material is polyvinyl chloride; and b. said additive material is a plasticizer for said polymeric material.
 47. The method according to claim 46 wherein: a. said polymeric material is low density polyethylene; and b. said additive is a wax. 